3070
The Journal of Physical Chemistty, Vol. 83, No. 24, 1979
analogous to those shown in Figure 3 for the decomposition of the hydroxylamines formed in the reactions involving primary and secondary amines. This heuristic model offers explanations for both the unusually great importance of the R-loss route in the 0 + TMA reaction and the fact that the H20loss route was not observed in the same reaction. The latter route would require the loss of two primary hydrogens in sequential steps, each of which involves a competition with a second pathway which is probably energetically favored. These six studies have begun to reveal details of the mechanism of 0 amine reactions under essentially collision-free conditions following the formation of an energy-rich adduct. Recognizing that an excited amine N-oxide is the first intermediate in this reaction, we have shown in this study that amine N-oxides with 60-70 kcal/mol of internal energy decompose not only along the path of lowest free-energy increase, but to a very great extent by other routes which have not been observed before.
+
Acknowledgment. The authors gratefully acknowledge the financial support of the National Science Foundation.
References and Notes (1) R. J. CvetanoviE, Adv. fhotochem., 1, 115 (1963). (2) R. E. Huie and J. T. Herron, "Progress in Reaction Kinetics", Vol. 8, Part 1, K. R. Jennings and R. B. Kundall, Ed., Pergammon Press, Oxford, 1975. (3) J. R. Kanofsky and D. Gutman, Chem. Phys. Lett., 15, 236 (1972); J. R. Kanofsky, D. Lucas, and D. Gutman, Symp. (Int.) Combust., [ f r o c . ] , 14th, 1972, 285 (1973).
Martin et al. (4) I. R. Slagle, R. E. Graham, and D. Gutman, Inf. J. Chem. Kinel., 8, 451 (1976). (5) J. R. Kanofsky, D. Lucas, F. Pruss, and D. Gutman, J. Phys. Chem., 78, 311 (1974). (6) K. Kirchner, N. Merget, and C. Schmidt, Chem. Ing. Techn., 46, 661 (1974). (7) R. Atkinson and J. N. Pitts, J . Chem. Phys., 68, 911 (1978). (8) Internal energy calculated assuming that the heat of formatlon of R,NOH is the same as that of monomethylamine (-12.0 kcal/mol) which was obtained from S. W. Benson, F. R. Cruickshank, D. M. Golden, G. R. Haugen, H. E. O'Neal, A. S. Rodgers, R. Shaw, and R. Walsh, Chem. Rev., 69, 279 (1969). (9) I.R. Slagle, J. F. Dudich, and D. Gutman, Chem. Phys. Lett., 61, 620 (1979). (10) R. E. Graham and D. Gutman in "Dynamic Mass Spectrometry", Vol. 5, D. Price and J. F. J. Todd, Ed., Heyden, London, 1978, p 156. (11) The lamp gases and their resonance energies (in eV) are Xe (8.Q CI (9.1), 0 (9.6), H (10.2), and Ar (11.6). (12) M. S.Foster and J. L. Beauchamp, J . Am. Chem. Soc., 94, 2425 (1972). 13) G. Hvistendahl and K. Undheim, Org. Mass. Spectrom.,3,821 (1970). 14) P. A. S.Smith, "The Chemistry of Open-Chain Organic Nitrogen Compounds", Vol. 11, W. A. Benjamin, New York, 1966, Chapter 8. 15) In DMA the C-N bond strength is 94.6 kcal/mol while the C-H bond strength is estimated to be 90.6 kcal/mol. Bond strengths are calculated from the known heats of formation and an estimation of AHACH,NHCH2) = 34 kcal/mol. S.W. Benson, "Thermochemical Kinetics", 2nd ed, Wiley, New York, 1976. 16) S.N. Foner and R. L. Hudson, J . Chem. Phys., 49, 3724 (1968); 53, 4377 (1970). Confirmation of these resuk reported by M. Gehring, K. Hoyermann, H. Gg. Wagner, and J. Wolfrum, Ber. Bunsenges. Phys. Chem., 73, 956 (1969). (17) S.Dayagl and Y. Degani, "The Chemistry of the Carbon-Nitrogen Double Bond", S.Patai, Ed., Interscience, New York, 1970, Chapter 2. (18) A. C. Cope and E. R. Trumbull, "Organic Reactions", Vol. 11, R. Adams, V. Boekelheide, T. L. Cairns, A. C. Cope, D. Y. Curtin and C. Niemann, Ed., Wiley, New York, 1960, Chapter 5.
Evaluation of the Olefinic Double Bond Influence in the Unimolecular Homogeneous Gas Phase Elimination of Alkenyl Acetates Ignaclo Marfin, Jose A. Hernhdez A.,+ Alexandra Rotlnov, and Gabriel Chuchanl" Centro de Gdmica, Instltuto Venezolano de Investigaciones Cienthas, Apartado 1827, Caracas, Venezuela (Received February 13, 1979; Revised Manuscript Received August 20, 1979) Publication cost assisted by Instituto Venezolano de Investigaciones Ciendficas
The rate coefficients for gas-phase pyrolyses of five alkenyl acetates have been measured in a static system over the temperature range 240-420 "C and pressure range 44-282 mmHg. The rate coefficients are expressed by the following Arrhenius equations: for 3-buten-1-yl acetate, log h(s-') = (13.20 & 0.17) - (200.8 f 2.1) kJ mol-l (2.303RT)-'; for 4-penten-1-yl acetate, log k(s-') = (12.81 i0.36) - (204.0 f 4.5) k J mol-1 (2.303RT)-l; for 5-hexen-1-ylacetate, log k(s-') = (12.43 f 0.14) - (197.5& 1.8) kJ mol-' (2.303RT)-'; for 1-penten-4-ylacetate, log k(s-l) = (12.34 f 0.25) - (178.2 f 2.9) kJ mol-l (2.303RT)-'; and for 2-methyl-4-penten-2-ylacetate, log k(s-') = (13.59 f 0.30) - (169.9 f 2.9) k J mol-' (2.303RT)-'. Steric acceleration seems to be a factor which slightly enhances the elimination when the CH2=CH substituent is interposed by at least three methylene groups with respect to the Cm-0 bond of the ester. However, the vinyl substituent adjacent to the carbon of ethyl acetate causes an appreciable increase in the rate of pyrolysis. The result is adequately explained on the basis of allylic weakening of the C6-H bond. Several arguments are presented to indicate that neighboring double bond group participation is improbable. Such a consideration comes from the fact that the transition state of ester pyrolysis is not very polar.
Introduction There are few works describing the fact that the olefinic double bond in the alcoholic part of acetates appears to anchimerically assist the molecular elimination of esters +Visitingscientist, Faculty of Medicine, Universidad Central de Venezuela, Caracas 0022-3654/79/2083-3070$01 .OO/O
in the gas The gas-phase pyrolysis of cyclohexen-1-ylethyl acetatel giving 1-vinylcyclohexene and spiro[2.51oct-4-ene (reaction 1) has suggested that the production of the spire compound may arise by the formation of a carbonium ion. This positively charged carbon atom is then stabilized by neighboring olefinic double bond participation; notwithstanding, the temperature depen0 1979 American
Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 24, 1979 3071
Gas Phase Elimination of Alkenyl Acetates rn
7 4% 85%
26% 14% (1)
dence of the composition of the product leads to some parallel reactions. Moreover, the rate coefficient of cyclohexen-l-ylethyl acetate was found to be approximately 300 times greater than ethyl acetate at 380 " C . However, these results may be the consequence of surface catalysis during decomposition in the stirred flow pyrolysis system. Thies and Schick2 were able to obtain rearrangement products by pyrolyzing the acetate of 4-cycloheptene-lmethanol (reaction 2). Apparently, the neighboring dou-
12.5%
87.5%
ble bond group seems to participate in product formation. These authors demonstrated the effect of surface catalysis in the stirred flow reaction by deactivation with allyl bromide, which decreased the rate of rearrangements by a factor of approximately 100. Further, 4-cyclohepten-lylmethyl acetate strikingly yields, in sealed ampules, a normal elimination to methylenecyclohept-4-ene together with a Cope rearrangement isomer in a ratio of 1:8, respectively (reaction 2). Therefore, it was concluded that flow conditions apparently enhance the surface-catalyzed rearrangements leading to 7r-route products at the expense of the simple elimination pathway. Although the catalyzed reactions do not occur in ampules, these pyrolyses were carried out in the gas phase and not in the liquid phase4 (10 mg of the acetate in 10-mL ampules gives a pressure about 235 mmHg). The strikingly different results shown in (2) reveal that, during pyrolysis in the stirred flow reactor, the concentration of the ester was low, and the surface of the reactor was rather large; whereas in ampule pyrolysis, the substrate was high in concentration while the surface was relatively small. Under a true gas-phase r e a c t i ~ nKramer ,~ and Smith3 reported the anchimeric assistance of the olefinic double bond in the norborene system (Ia-c). This claim involves -HA
CHA
Ia
CHA
Ib
IC
a slight difference in the relative rate. (The reported relative rates at 343 "C for Ia-c were 1.0, 0.87, and 1.9, respectively.) Unfortunately, the products could not be analyzed which causes some doubts with these highly strained bicyclic molecules, since changes of strain could
yield different results than those reported. Recent works regarding anchimeric assistance in gasphase pyrolysis of 2-substituted ethyl chloridesk7 suggested that neighboring group participation is to be expected under these conditions when7 (a) the transition state is highly polar, (b) the participating atom is large and can therefore overlap, or (c) when the participating atom is highly polarizable. However, the transition state for ester pyrolysis is less polar than halide pyrolysiss which implies that neighboring group participation in the former compounds will be more difficult to occur but not improbable. The literature describes 7r-bond participation during the solvolysis of simple allylcarbinyl tosy1ate"l' type of compounds. By analogy, it seemed interesting to study, in the present work, the gas phase pyrolysis of alkenyl acetates (reaction 3) and to redetermine the kinetic of the reactions (4)12J3and (5)13under similar static experimental conditions for comparative purposes. Furthermore, it was also the aim of this research to find out whether olefinic double bonds are capable in assisting the gas-phase elimination of esters.
Experimental Section The reagents 3-buten-l-yl acetate and 4-penten-l-yl acetate were obtained from Aldrich. The acetates of 5hexen-1-01 (K & K Lab.) and l-penten-4-01 (Pflatz and Bauer) were prepared when these alcohols were treated with acetyl chloride as reported.14 5-Hexen-l-yl acetate had a boiling point of 69 "C at 16 mmHg (lit. bp 169-170 "C at 704 mmHg15). l-Penten-4-yl acetate had a boiling point of 125 " C a t 620 mmHg (lit. bp 133 "C a t 743 mmHg16). 2-Methyl-4-penten-2-yl acetate was prepared by acetylating 2-methyl-4-penten-2-01 (Pflatz and Bauer) with ketene as described17 (bp 48 "C at 24.5 mmHg; lit. by 48-48.5 "C a t 24.5. mmHg13). These esters were distilled several times and the fraction with over 99.0% purity (gas-liquid chromatography) was used. Butadiene was acquired from Matheson, 1,4-pentadiene and 1,5-hexadiene were from Aldrich, and cis-1,3-pentadiene, trans-1,3-pentadiene, 4-methyl-l,3-pentadiene, and 2-methyl-1,4-pentadiene were from K & K Lab. These compounds were over 99.070 pure and were used as standard references. Gasliquid chromatographic analyses were done with a Hewlett-Packard 5700A, Varian Aerograph 1400, and PerkinElermer F-11, Apiezon L 20%-Chromosorb P A.W. 60-80 mesh and Porapak Q columns were used to quantitatively determine 3-buten-l-yl acetate and butadiene, respectively. A column of FFAP 7%-Chromosorb W A.W. D.M.C.S. 80-100 mesh was used for 4-penten-l-yl acetate, 5-hexenl-yl acetate, l-penten-4-yl acetate, 2-methyl-4-penten-2-yl acetate, 1,4-pentadiene, and 1,5-hexadiene. Finally, a column of 12-ft bis(2-methoxyethyladipate) 20 7'0 -Chromosorb P 80-100 mesh was used for cis-1,3-pentadiene and trans1,3-pentadiene analyses, whereas a column of diisodecyl phthalate 5%-Chromosorb G A.W. D.M.C.S. 60-80 mesh was used for 4-methyl-l,3-pentadiene and 2-methyl-1,4pentadiene. The purity of the esters and of the diene products was verified with a Hitachi-Perkin-Elmer RMU-6H mass spectrometer and by infrared and nuclear magnetic resonance spectroscopy. The least-squares calculations were carried out with a Digital PDP 1145 computer. The kinetics were followed manometrically in a conventional static system. The reaction vessel was seasoned with the product of allyl bromide decomposition.1sJ9 Measurements at different points along the reaction vessel showed no temperature gradient. The temperature was found to be constant within f0.2 "C with a calibrated platinumplatinum-13% rhodium thermocouple. The substrate was
3072
The Journal of Physical Chemlstry, Vol. 83, No. 24, 1979
TABLE I: Variation of the Rate Coefficient with Temperature acetate
temp, "C 104h,s - *
3-buten-1-yl acetate
4-penten-1-yl acetate
5-hexen-1-yl acetate
1-penten-4-yl acetate
2-methyl-4-penten-2-yl acetate
340.0 350.1 359.9 365.0 370.0 375.1 380.1 380.1 390.1 395.1 400.0 405.1 410.1 420.1 369.7 380.0 389.9 394.8 399.9 410.0 420.1 330.0 340.3 350.1 359.8 377.5 240.1 250.1 260.2 270.1 28a.i
1.25 2.28 4.14 5.79 7.80 10.45 13.71 5.12 8.91 11.76 15.41 20.34 27.05 43.82 2.44 4.20 7.26 9.41 12.76 20.85 35.53 7.96 14.33 24.84 43.27 105.47 1.94 4.28 8.49 17.43 35.18
injected directly into the reaction vessel with a syringe through a silicon rubber septum.20 Results The gas-phase pyrolysis of alkenyl acetates, in vessel seasoned with the product of the decomposition of allyl bromide, is described by reactions 3-5. C€I3COOCH2(CHZ),CH=CH2 n = l n=2 n=3 CH2=CH(CH,),CH=CHz CH3COOH (3) n=O n = l n=2 CH3C(OOCCHJHCH2CH=CH2 CH3COOH CH2=C€ICHZCH=CH2 CH&H=CHCH=CH2 (4) cis and trans
-
+
+ +
+
(CH3)~C(OOCCH3)CH2CHICII~CH3COOH + CH2=C(CH3)CHZCH=CHz
(CH,q)&=CHCH=CH2 (5) The stoichiometry of reactions 3-5 has been examined by determining the ratio of the final pressure, Pf,to the initial pressure, Po. The average experimental Pf/PO values at four different temperatures and ten half-lives were as
Martin et al.
follows: 3-buten-1-yl acetate, 1.72; 4-penten-1-yl acetate, 1.99; 5-hexen-1-ylacetate, 2.03; 1-penten-4-ylacetate, 1.97; and 2-methyl-4-penten-2-yl acetate, 1.96. The departure of Pf = 2P0for 3-buten-1-yl acetate results largely from the polymerization of the olefinic product butadiene. However, the verification of the stoichiometry for reactions 3-5, up to 35% of reaction for 3-buten-1-yl acetate and up to 70% of reaction for the other four esters, is confirmed by comparing the percentage decomposition of the acetates from pressure measurements with those obtained from chromatographic analysis of the olefinic dienes and also with acetic acid titration with 0.05 N sodium hydroxide solution. The homogeneity of the reactions was checked by using a vessel with a surfaces-to-volume ratio of 6.14 relative to the normal vessel. The packed and unpacked clean Pyrex vessel had only a slight heterogeneous effect on 3-buten1-yl acetate. However, when the vessels are seasoned with allyl bromide no significant effect on the rate coefficients for elimination of the esters was observed. The presence of propene, a free-radical inhibitor, had no effect on the rates and no induction period was observed. The rate coefficient for pyrolyses of these esters are independent of the initial pressure and the fust-order plots are satisfactorily linear up to 35% for 3-buten-1-yl acetate and up to 70% for the other acetates. The variation of the rate coefficient with temperature is shown in Table I. The experimental data of Table I were fitted to the following Arrhenius equations, where 80% confidence limits being quoted 3-buten-1-yl acetate log kl(~-') = (13.20 f 0.17) (200.8 f 2.1) kJ moll1 (2.303RT)-l 4-penten-1-yl acetate log k l ( ~ - ' )= (12.81 f 0.36) (204.0 f 4.5) kJ mol-' (2.303RT)-' 5-hexen-1-yl acetate log kl(s-') = (12.43 f 0.14) (197.5 f 1.8) kJ mol-' (2.303RT)-l 1-penten-4-yl acetate log kl(s-') = (12.34 f 0.25) (178.2 f 2.9) k J mol-l (2.303RT)ll 2-methyl-4-penten-2-yl acetate log ki(s-') = (13.59 f 0.30) (169.9 f 2.9) kJ mol-' (2.303R77-l
D~sCussion The effect of a vinyl substituent along the alkyl part of alkenyl acetates is shown in Table 11. The comparative data in Table XI indicates that, when the vinyl substituent
TABLE 11: Kinetic Parameters of Alkenyl Acetates a t 380 C 1 0 4 k , , re1 rate acetate system" s-' per H
E,, kJ/mol
log A, s - '
ref
CH,CH, 0Ac S 3.24 1.0 200.5(* 3.8) 12.55(* 0.30) 2lb>" 13.20($0.17) this work 6.4 200.8( i 2.0) S 13.80 CH,=CHCH,CH,OAc 12.2 2 2d 2.2 194.1 f 4.68 CH,CH,CH,CH,CH,OAc 13.00(i0.17) this work 2.4 203.6(t 2.1) S 5.25 CH,= CHCH,CH,CH,OAc 12.54 23 1.4 200.8 f 3.02 CH,CH,CH,CH,CH,CH,OAc 12.43(+0.14) this work 2.0 197.5( i 1.8) S 4.27 CH, = CHCH,CH,CH,CH,QAC " s = static, f = flow. The Arrhenius parameters were recalculated from data of the reference by the least-squares proData taken as the preferred value, see ref 30. d Values obtained from ref cedure. The confidence coefficient was 0.8. 24.
The Journal of Physlcal Chemistry, Vol, 83, No. 24, 1979 3073
Gas Phase Elimination of Alkenyl Acetates
a t 330 C TABLE 111: Comparative Arrhenius Parameters and Relative Rates for P-Vinyl Substituents __ 104k,, re1 rate acetate svstemQ s" Per H E , . kJlnio1 log A , s-' 12.55(+0.30) S 0.15 1.0 200.5(+3.8) CH,CH,OAc 12.50(?: 0.36) 1.7 199.3(+4.6) S 0.17 CH,CH,CH,CH,OAc CH, = CHCH, CH, OAc S 0.64 6.4 200.8(+2.1) 13.20(+0.17) 1.0 191.2(?:1.7) 13.20(r0.20) S 4.37 CH,CH(OAc)CH, 1.3 188.7(+1.7) 13.00(t0.10) S 4.57 CH, CH, CH, CH( OAc)CH, 2.2 186.5(+2.5) 13.06(+0.23) S 8.13 CH,=CHCH,CH(QAc)CH, 2.1 186.1(i2.5) 13.00(+0.10) f 7.59 12.03 2.1 174.8 S 7.76 12.34(+0.25) 2.2 178.2(+2.9) S 8.13 CH,CH(OAc)C( CH,), CH,CH,CH( OAc)C(CH, ), CH,=CHCH,CH(OAc)C(CH,),
S S S
CH3C(0Ac)(CH3)2 CH,CH,C(OAc)(CH,), CH,=CHCH,C(OAc)( CH,),
S S S S
3.72 6.61 21.88 457.1 691.8 436.5 741.3
1.0
ref 21byC 2lblC this work 26 27 12' 27 13 this work
2.7 8.8
184.8(+3.7) 187.3(+1.6) 194.0(+2.5)
12.58(+0.33) 13.04(+0.13) 14.14(+0.19)
28b 28' 2gb
1.0 1.7 1.1 1.8
167.8(+2.7) 168.4(+0.5) 172.3 169.9(+2.9)
13.19(+0.27) 13.43(+0.05) 13.56 13.59(+0.80)
30b,C 30b 13 this work
a s = static, f = flow. The Arrhenius parameters were calculated from data of the cited reference by the least-square proData taken as the preferred value, see ref 24. cedure. The confident coefficient was 0.8.
is interposed by at least three methylene groups with respect to the C,--0 bond, steric acceleration seems to be responsible for a slight rate enhancement. Such a point of view has already been advanced in a recent communication regarding the effect of alkyl groups and several polar substituents in the gas-phase pyrolysis of @-substituted ethyl acetates.z5 Yet, the CH2=CH substituent a t the @ carbon in 3-buten-1-yl acetate causes the rate of elimination to be significantly greater by a factor of 6.4 more than ethyl acetate (Table 11) and 3.8 greater than the corresponding saturated ester butyl acetate (Table 111). According to these estimated rate ratios, the vinyl substituent a t the P carbon of ethyl acetate does not seem to sterically enhance the pyrolysis reaction. The point of the logarithm of the relative rate of this ester with respect to ethyl acetate against its E, value is beyond the correlation line reported beforeSz5Consequently, another factor must be operating during the process of elimination. An interesting coincidence with the results of the present work is that 3-buten-1-yl tosylate solvolyze in 98% formic acid 3.7 times greater than n-butyl t ~ s y l a t ea, ~fact attributed to neighboring group participation assisting in the reaction. The similarity in rate ratios between the gasphase elimination of 3-buten-1-yl acetate with the solvolysis of 3-buten-1-yl tosylate led to the examination of the gas phase pyrolysis of esters containing vinyl substituents adjacent to the @-carbonatom in allyl carbinyl system (11). H
0
II CH3-C
H'C-CH=CH;! I I
C-R \R,
I1
The kinetic parameters for the thermal decomposition of several esters of type I1 are listed in Table 111. The pyrolysis experiments by different workers usually give slightly distinct Arrhenius parameters because of subtle differences in gas-phase techniques. However, many a time, dissimilar values of log A and E, may compensate, thus giving closely reproducible rate coefficients. An example of this is the result from four different pyrolysis studies of 4-penten-2-yl acetate shown in Table 111. Each type of compound with the vinyl substituent adjacent to the @-carbonatom is compared with the corresponding unsubstituted and saturated acetates, respec-
tively, Further, it is interesting to note that when the direction of elimination is exclusively toward the CB-H bond adjacent to the vinyl substituent, such as in 3-buacetates, the rate ten-1-yl and 2,2-dimethyl-5-hexen-3-yl of elimination is significantly higher. Otherwise, when the direction of elimination takes place simultaneously toward another C,-H, besides the one adjacent to the vinyl substituent, the rate of the reaction has a relatively small increase. This is seen in 1-penten-4-yl and 2-methyl-4penten-2-yl acetates. Prior to analyzing these observations it becomes important to determine if the olefin distribution in these cases is subject to kinetic or thermodynamic control. The yields of olefins produced from these esters a t different temperatures and under the same working conditions of our laboratory are given in Table IV. The product distribution results from 1-penten-4-yl and 2-methyl-4penten-2-yl acetates are quite similar with those reported in the literature.l2J3 Additional data concern whether the rate of formation of these olefins varies or not as the reaction progresses a t a given temperature. The data of Table V confirm the results shown in Table IV indicating that the formation of olefins from gas-phase elimination of these acetates proceeds by kinetic control. Analyses of Tables IV and V make possible the assessment of the effect of olefinic double bond toward its adjacent C,-H bond. Therefore, by logical inference, Table VI indicates from a detailed calculation of partial rates that there is a more pronounced effect of the vinyl substituent toward the elimination of its adjacent C,-H bond. The relative rates toward the C,-H next to the olefinic double bond (Table VI) insinuate an allylic weakening of the @ hydrogen and, consequently, a faster rate is observed. This means the neighboring vinyl group does not provide anchimeric assistance. A reasonable support of the above consideration comes from the fact that the rate of pyrolysis of the phenethyl acetatez1 is 1.2/1 greater than rate for 3-buten-1-yl acetate a t 330 "C. The benzylic C,-H is known to be more acidic than allylic C ~ H . ~Further* I ~ ~ more, the phenyl group in PhCHzCH20Acwas not found to participate in the gas-phase elimination of this ester.6-z1 On the other hand, if the CH2=CH group were to assist the reaction, then 3-buten-1-yl acetate should have a considerably greater rate than phenethyl acetate (u+CH2=CH > a+ph).32338 Yet, the result is quite the contrary. The present observation confirms that the high polarity of the transition state for gas-phase pyrrolysis of organic mole-
3074
The Journal of Physical Chemistry, Vol. 83, No. 24, 1979
Martin et al.
TABLE IV: Distribution of Olefins from Pyrolysis of Alkenyl Acetates at Different Temperaturesa acetate
temp, C
AcO CH, C H ,CH= CH,
350.1 359.9 370.0 380.1 AcOCH,CH,CH,CH=CH, 390.1 400.0 410.1 420.1 AcOCH,CH,CH,CH,CH=CH, 369.7 389.9 410.0 420.1 'CH, CH( AC0 )CH, CH= C H , 330.0 340.3 350.1 359.8 (CH,),C(AcO)CH,CH=CH, 240.1 260.2 270.1 280.1 (CH,),CCH(OAC)CH,CH=CH,~ 296.3 316.2 319.1
products
yield, %
CH,=CHCH=CH,
98.4 97.8 98.1 97.4 99.2 99.6 99.4 99.3 99.3 99.1 99.4 98.8 23.5, 20.2, 55.5 25.9, 20.7, 53.4 24.3, 19.3, 56.4 25.3, 19.8, 54.8 50.7, 44.7, 4.6 51.4, 45.7, 3.0 49.2, 46.2, 4.6 50.7; 45.9; 3.4 5.3, 94.7 5.1, 94.9 5.2, 94.8
CH,= CHCH,CH= CH,
CH,=CHCH,CH,CH=CH,
CH,=CHCH,CH=CH,, cis-CH,CH=CHCH=CH,, trans-CH,CH= CHCH=CH, CH,=C(CH,)CH,CH=CH,, (CH,),C=CHCH=CH,, CH,=C( CH,)CH= CHCH, &(CH,),CCH=CHCH=CH,, trans-(CH,),CCH=CHCH=CH
a The chromatographic product analysis from 3-buten-1-yl acetate pyrolysis was made up to 35% reaction. A small amount of butadiene product polymerizes. The product of the other esters were analyzed up t o 70% reaction. Data from ref 35.
TABLE V: Variation of Diene Formation from Percentage Pyrolysis of the Acetates at One Temperature decomp, temp, o/c "C
ester AcOCH,CH,CH=CH,
9 16 22 31 15 26 41 62 17 27 43 61 16 24 45 64 17 29 51 60
AcOCH,CH,CH,CH=CH,
AcOCH,CH,CH,CH,CH, =CH,
CH,CH( OAc)CH,CH=CH,
(CH, ), C(0 AC)CH, CH= CH,
CH,=CHCH=CH,
410.1
CH,=CHCH,CH=CH,
410.0
CH, =CHCH,CH,CH=CH,
350.1
CH,=CHCH,CH=CH,, cis-CH,CH=CHCH=CH,, trans-CH,CH=CHCH=CH,
280.1
CH,=C(CH,)CH,CH=CH,, (CH,),C=CHCH=CH,, CH,=C(CH,)CH=CHCH,
TABLE VI: Rate of Elimination per Branch at 330 C left branch 104k,, yield, % 100 100 100 50 45 65 74 75.7 100
100 100 33.3 24 50 45.6
S-'
re1 rate per H
acetate
0.15 0.18 0.64 2.19 2.06 5.28 5.74 6.15
1.0 1.8 6.4 1.0
3.72 6.61 21.88
1.0 2.7 8.8
CH ,CH( OAc)C( CH, ), CH,CH,CH( OAc)C( CH,), CH,=CHCH,CH(OAc)C(CH,),
1 .O 1.7 2.2 3.4
CH,C(OAc)(CH, ), CH,CH,C(OAc)(CH,), CH,=CHCH,C( OAc)(CH,),
150.8 166.0 218.3 338.0
1.4
3.6 3.9 4:2
yield, %
products
370.0
right branch yield, %
104h,, s-'
re1 rate per H
CH,CH,OAc CH,CH,CH,CH,OAc CH, = CHCH,CH,OAc CH,CH(OAc)CH, CH,CH,CH,CH( OAc)CH, CH,=CHCH,CH(OAc)CH,
97.7 98.2 98.6 97.9 99.6 99.3 99.3 99.5 99.2 99.6 99.2 99.5 24.3, 19.3, 56.5 22.9, 20.0, 57.1 25.3, 19.0, 55.7 24.6, 19.2, 56.2 50.7, 45.8, 3.4 50.9, 45.8, 3.3 51.1, 45.9, 3.0 50.0, 46.2, 3.7
ref 21 21 this work
50 55 35 26 23.5
2.19 2.51 2.85 2.02 1.91
1.0 1.2 1.3 0.9 0.9
26 27 12 13 this work 28 28 29
66.6 76 50 54.4
301.7 525.8 218.3 403.3
1.0 1.7 0.7 1.3
30 30 13 this work
The Journal of Physical Chemistry, Vol. 83, No. 24, 1979 3075
Photochemistry of Silicon Compounds
References and Notes
03
02
01
00
01
02
03
04
05
IT*
Figure 1. Plot of log k,e,values for pyrolysis of CH,=CHCH,CH(OAc)R against u*.
cules is of utmost importance for neighboring group part i ~ i p a t i o n .Since ~ the transition state for ester pyrolysis is semipolar or semiconcerted3J3~21~25~26~34~35 the occurrence of anchimeric assistance from substituents is therefore difficult. The participation found in reaction 1and 2 is, as is stated by the authors,lZ due to formation of a positive carbon atom stabilized by surface catalysis during the stirred flow pyrolysis of the system. An assessment of the effect of substituent R in the elimination of the ester type I11 toward the Co-H adjacent CH,=CHCH,CH-R OAc
IIIa, Z = H IIIb, Z = CH, IIIc, Z = (CH, ) 3 C to the vinyl group is plausible, since olefin formation is found to be produced by kinetic control (Tables IV and V). Therefore, the effect of R at the a carbon in I11 gives a good linear relationship with u* values (Taft equation)36 (Figure 1). Consequently the alkyl substituents at the a carbon in the butenyl acetates seems to exert a +I electron release and not steric acceleration.21
Hanack, M.; Schneider, H. H.; Schneider-Bernlohr, H. Tetrahedron 1967, 23, 2195. Thies, R. W.; Schick, L. E. J . Am. Chem. SOC. 1974, 96, 456. Kramer, D. J.; Smith, G. G. Int. J. Chem. Kinet. 1974, 6 , 849. Wertz, D. H.; Allinger, N. L. J . Org. Chem. 1977, 42, 698. Chuchani, G.; Martin, I.; Bigley, D. B. Int. J. Chem. Kinet. 1976, 10, 649. Herndndez, J. A.; Chuchani, G. Int. J . Chem. Kinet. 1976, 10, 923. Chuchani, G.; Martin, I.; Martin, G.; Bigley, D. B. Int. J. Chem. Kinet. 1979, I f , 109. Maccoll, A. Chem. Rev. 1969, 69, 33. Servis, K. L.; Roberts, J. D. J . Am. Chem. SOC.1964, 86, 3773. Rogan, J. B., J . Org. Chem. 1962, 27, 3910. Kosower, E. M. “An Introductionto Physical Organic Chemistry”; Wiley: New York, 1968; p 120, and references cited therein. Emovon, E. U.; Maccoll, A. J . Chem. SOC.1964, 227. Lum, K. K.; Smith, G. G. Int. J. Chem. Kinet. 1969, 1 , 401. Sarel, S.;Newman, M. S. J. Am. Chem. SOC.1956, 78, 5416. Golinov, V. P. Zh. Obshch. Khim. 1952, 22,2131. See Chem. Abstr. 1954, 48, 1240c. Wagner, G. Berichte 1694, 27, 2434. Vogel, A. I.“Practical Organic Chemistry”, 3rd ed;Longmans: London, 1956; p 372. Maccoll, A. J . Chem. SOC. 1955, 965. Maccoll, A.; Thomas, P. J. J . Chem. SOC. 1955, 979. Bridge, M. R.; Davies, D. H.; Maccoll, A.; Ross, R. A,; Banjoko, 0. J . Chem. SOC.B 1966, 805. Burgh Norfolk, S.de; Taylor, R. J. Chem. Sm.,Perkin Trans. 2 1976, 280. Scheer, J. C.; Kooyman, E. C.; Sixma, F. L. J. Recl. Trav. Chim. Pays-Bas 1963, 82, 1123. Ruzicka, V.; Cerveny, L.; Prochazka, J. Inst. Chem. Techno/.Prague, Org. Chem. Techno/. 1970, C15, 57. Benson, S. W.; O’Neal, H. E. Natl. Stand. Ref. Data Ser., Nat/. Bur. Stand. 1970, No. 21. Chuchani, G., Martin, I.; Avila, I. Int. J . Chem. Kinet. 1979, 11, 561. Taylor, R. J. Chem. Soc., Perkin Trans. 2 1975, 1025. Chytry, V.; Obereigner, B.; Lim, D. Europ. Polym. J . 1973, 9 , 493. Chuchani, G.; Martin, G.; Barroeta, N.; Maccoll, A. J . Chem. Soc., Perkin Trans. 2 1972, 2239. Chuchani, G.; Piotti de Chang, S.; Lombana, L. J. Chem. Soc., Pekin Trans. 21973, 1961. Emovon, E. U.; Maccoll, A. J . Chem. SOC.1962, 335. House, H. 0. “Modern Synthetic Method”, 2nd ed; Benjamln: Philippines, 1972; Chapter 9. Brown, R. S.; Taylor T. G. J . Am. Chem. Sac. 1973, 95, 8025. Johnson, C. D. “The Hammett Equation”, Cambridge: London, 1973; Chapter 2. Taylor, R. J . Chem. SOC.,Perkin Trans. 2 1972, 165. Cuenca, A.; Chuchani, G. Int. J . Chem. Kinet. 1977, 9 , 379. Taft, Jr., R. W. “Steric Effect in Organic Chemistry”, Newman, M. S., Ed.; Wiley: New York, 1956; Chapter 13.
Photochemistry of Silicon Compounds. 6. The 147-nm Photolysis of Tetramethylsilane L. Gammle,’ C. Sandorfy,’ and 0. P. Sfrausz* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2, and Dgpartement de Chimie, Universit6 de Montreal, Montreal, Quebec, Canada H3C 3Vf (Received December 28, 1978; Revised Manuscript Received August 10, 1979) Publication costs assisted by the University of Alberta
The 147-nm gas-phase photolysis of tetramethylsilane yielded ten measurable and several trace products along with a solid deposit. From the effect of pressure, exposure time, deuterium labeling, and added nitric oxide on the quantum yields of individual products, the following primary steps were postulated: CH3 + Si(CH3)3 (4 = 0.43); 2CH3 + Si(CH3I2(4 = 0.24); CH4 + CH2Si(CH3I2(4 = 0.17); CH3 + H + CH2Si(CH3)2(4 = 0.10); Hz + CHSi(CH3)3(4 = 0.02); and CH2 + (CH3I3SiH(4 = 0.04). Fluorescence could not be observed and the The secondary reactions of the principal silicon radicals Si(CH3),, CH2Si(CH3)2,and upper limit for +f is Si(CH3)2and the mechanism of the nitric oxide inhibited reaction are discussed and it is shown that the siloxy radical (CH3I3SiOcan displace CH3 from the substrate to give hexamethyldisiloxane. In earlier studies3+ on the 147- and 124.6-nm photolysis of monomethylsilane and dimethylsilane it has been shown *Address correspondence to this author at the University of Alberta. 0022-3654/79/2083-3075$01 .OO/O
that the photochemical decomposition of these molecules is characterized by a complex array of simultaneous, competing primary steps comprised of molecular and free-radical modes of splitting, Yielding two Or more fragments. The role of polyfragmentation is more important in the 0 1979 American
Chemical Society
